Alkyl or alkaryl hydroxamic acids and their salts are well-known collectors for the froth flotation of oxide minerals. Soviet workers have found a variety of applications for such alkyl hydroxamic acids. A recent review summarizes the flotation application of alkyl hydroxamic acids (Pradip and Fuerstenau, "Mineral Flotation with Hydroxamate Collectors", in "Reagents in the Minerals Industry", Ed. M. J. Jones and R. Oblatt, Inst. Min. Met., London, 1984, pp. 161-168). Hydroxamic acids have been used for the flotation of metals or minerals such as pyrochlore (Nb, Ta), fluorite, huebnerite, wolframite, cassiterite, muscovite, phosphorite, hematite, pyrolusite, rhodonite, chrysocolla, malachite, barite, calcite, and rare-earths. They are generally more powerful and more selective then conventional fatty acids, fatty amines, petroleum sulfonates and alkyl sulfates. However, the commercially employed methods of making alkyl or alkaryl hydroxamic acid or its salts are tedious, and unsafe from the point of view of industrial production. For example, Organic Synthesis, Vol. II, page 67 sets forth a procedure for making potassium alkyl hydroxamate wherein a methanol solution of KOH (56 gm in 140 cc of methanol) and another of NH.sub.2 OH HCl (41.7 gm in 240 cc of methanol) are combined. The KCl byproduct is filtered off. To the filtrate is added 56.1 gm of a mixed liquid of methyl caprylate/caprate. After standing 24 hours, the product crystals (50 gm, or 67% yield) are filtered off. A major drawback of this method is the use of a large amount of methanol which is toxic and flammable. Another drawback is the use of potassium hydroxide which is more expensive than sodium hydroxide. Furthermore, the filtration of methanolic reaction mixture on an industrial scale is obviously not desirable in terms of safety. Finally, the yields are quite low.
Hartlage (U.S. Pat. No. 3,922,872, Jan. 20, 1976) claims an improved method of making fatty hydroxamates. Dimethylamine is used to effect the reaction between hydroxylamine sulfate and the methyl ester of a fatty acid in an anhydrous lower alcohol slurry. The free hydroxamic acids formed are neutralized with dimethylamine or an alkali metal base to yield, after filtering and drying, the ammonium or alkali metal salt precipitate. The procedure given, however, still employs flammable lower alcohols, i.e., methanol, ethanol or isopropanol. Furthermore, because of the heterogeneous nature of the reaction, the reaction rate is very slow, e.g., 15 hours in methanol and 5 days in isopropyl alcohol and filtration of the final hydroxamate product is still necessary. In an operation when methanol, a toxic flammable liquid, is employed, a hazardous environment is created. Finally reported yields are only in the 75-76% range.
Various Russian workers have reported methods for making alkyl hydroxamic acids and/or their salts in aqueous alkaline media. Sodium alkyl hydroxamates were made by reacting the methyl ester of a C.sub.7-9 carboxylic acid with an aqueous solution of hydroxylamine sulfate and NaOH at a molar ratio of 1:1.22:2.2 and a temperature of 55.degree. C. or below (Gorlovski, et. al. Vses. Soveshch. po Sintetich. Zhirozamenitelyam, Poverkhnostnoaktivn, Veschestvam i Moyushchim Sredstvam, 3rd, Sb., Shebekino, 1965, 297-9 Chem. Abst. 66, 4983h, 1967). A yield of only 72-78% of the free C.sub.7-9 hydroxamic acid was reported by Shchukina et. al. (Khim. Prom., Moscow, 1970, 49(3) 220) by reacting one mole of the methyl ester, 1.45 mole hydroxylamine sulfate, 7.39-7.82 moles NaOH for two hours at 20.degree.-25.degree. C. and one hour at 55.degree.-60.degree. C., followed by acidification to pH 4-5 at temperatures below 40.degree. C. Again, in Sin. Primen. Novykh Poverkh. Veshchestv, 1973, 123-31 reported in C.A. 80, 1974, 95199K, Shchukina et al report a simple lab method for the production of a reagent designated as IM-50 from C.sub.7-9 esters. In a Russian Patent (U.S.S.R. No. 390,074, July 11, 1973 Chem. Abst. 79, 115162C (1973)) and also in an article (Zh. Prikl, Khim, (Leningrad) 1972 45(8), 1895-7, Chem. Abstract 78, 29193m 1973), Russian workers reported improved yields with the use of 3-5% of an anionic emulsifier in an alkaline aqueous medium. The authors reported that the use of an anionic surfactant such as sodium lauryl sulfate (3-5% based on the weight of the methyl ester), gave an improved yield of 61.2% for valerihydroxamic acid and 89% for caprihydroxamic acid. To obtain the yields claimed, however, a 40 molar % excess of hydroxylamine hydrochloride or sulfate was required. Furthermore, both the sodium salts and the free hydroxamic acids recovered are solids which are difficult to handle and process.
In another Russian patent (U.S.S.R. No. 513,970, May 15, 1976, Chem. Abst. 85, 66277g, 1976) a solution of mixed free C.sub.8-11 hydroxamic acids was obtained in hydrocarbons. This was achieved by treating the sodium alkylhydroxmates with a mineral acid in the presence of 100-250 weight percent of a hydrocarbon containing less than 20% polar organic components (e.g., higher alcohols or esters). The aqueous layer containing NaCl or Na.sub.2 SO.sub.4 was discarded as effluent.
Finally, U.S. Pat. No. 4,629,556 has recently issued wherein various colored impurities are removed from kaolin clays utilizing alkyl, aryl or alkylaryl hydroxamates as collectors. The hydroxamates are disclosed as having been produced by reacting free hydroxylamine with the methyl ester of an organic acid of appropriate hydrocarbon chain length and configuration in a non-aqueous medium such as methanol much in the same manner as taught in the above-mentioned articles.
While these reports certainly represent advancement of the art, there are still many drawbacks regarding industrial production. On a large scale of production, for example, the aqueous effluent can be substantial and can pose a serious problem for disposal. Furthermore, in order to obtain a product in liquid form, the alkali metal alkyl hydroxamates must be acidified to the free hydroxamic acids. This acidification is an additional step and causes a substantial increase in processing and handling time and costs. The use of anionic surfactants as taught by the Russians also causes a foaming problem during manufacture.